The present invention relates to an electronic control sewing machine which stores, as pattern signals, the amounts of needle bar amplitude and fabric feed, drives control motors per rotation of the sewing machine in response to the pattern signals, outputs rotations of the control motors via cam mechanisms, and produces stitching patterns, and more particularly relates to switching mechanisms of the control amounts of the electronic sewing machine.
For the control motor of the electronic sewing machine, a pulse motor of hybrid type is structurally suitable to control, since small stepping angles may be obtained thereby. However, machining process of high precision is required for obtaining the small stepping angle, for which high expense is a problem. On the other hand, a pulse motor of inductor type belonging to PM (permanent magnet) is produced at cheaper cost, but since the small stepping angles could not be obtained thereby due to its structure, it is not suitable for the control motor of the electronic control sewing machine.
With respect to the fabric feed amount in the control amounts in formation of stitching patterns by the electronic control sewing machine, such a fine feeding amount of about 0.25 mm is required for buttonhole stitching. For this object, it is practiced to divide 8 mm of the feed control range between a forward side 5 mm and backward side 3 mm into 38 steps.
In the amount of a needle bar amplitude, if the amplitude control range is divided into 24 steps for all of the patterns, such a division is sufficient.
In the electronic control sewing machine which outputs rotation of the control amount via the cam mechanisms, if the feed control range is divided into 32 steps and the amplitude control range is divided into 24 steps, and if the pulse motor of the inductor type of the stepping angle 7.5.degree. is used, the control ranges will be 32.times.7.5.degree.=240.degree. concerning the feed and will be 24.times.7.5.degree.=180.degree. concerning the amplitude.
Ordinary stitching patterns have moderate changings of the fabric feed amount and the needle bar amplitude amount. In an example, for instance, shown in FIG. 1, the maximum change of the fabric feed amount is from needle droppings (3)-(4) to needle droppings (4)-(5). Assuming that the needle droppings (3)-(4) are forward 5 steps and the needle droppings (4)-(5) are forward 12 steps, the changing amount is 7 steps (52.5.degree.). The maximum change of the amplitude amount is 8 steps between the needle droppings (7) and (8). There is no problem about positioning time of the control motor.
However, concerning the fabric feed amount, its changing amount is often large in special stitching patterns. For instance, in an example of feather stitchings shown in FIG. 2, the maximum change of the fabric feed amount is from the needle dropping (3)-(4) to the needle droppings (4)-(5). Assuming that the needle droppings (3)-(4) are the forward 12 steps and the needle droppings (4)-(5) are the backward 12 steps, the changing amount is 24 steps and 180.degree. in calculation. Therefore, such a special stitching pattern (called "large feed stitching pattern" hereinafter) takes a large time for the positioning of the control motor, and it is necessary to reduce the rotation speed of the main shaft of the sewing machine.
There is a special stitching pattern, the changing amount of which is large with regards to the needle amplitude amount. In zigzag stitching, for instance, shown in FIG. 3 where the needle drops at a left basic line L and a right basic line R, changings are made by 24 steps per each stitch (180.degree. in calculation). Therefore such stitchings (called "large amplitude stitching pattern" hereinafter) take to a large time for the positioning of the control motor, as seen in the feed, and it is necessary to reduce the rotation speed of the main shaft.
As having mentioned, the pulse motor of the inductor type is unsuitable for producing the large feed stitching pattern and the large amplitude stitching pattern, since the small stepping angles are not obtained.